Cooperative Extension Service
Purdue University
West Lafayette, IN

Herbicide Mode-Of-Action Summary

The mode-of-action is the overall manner in which a herbicide affects a
plant at the tissue or cellular level. Herbicides with the same mode-of-
action will have the same translocation (movement) pattern and produce
similar injury symptoms. Selectivity on crops and weeds, behavior in the
soil and use patterns are less predictable, but are often similar for
herbicides with the same mode-of-action.
This publication organizes herbicides into those which are applied to
foliage (many of these are applied to soil as well) and those herbicides
applied almost strictly to soil. The foliar applied groups are then
divided into three categories according to movement through the plant:

Each translocation group is subdivided into mode-of-action
groups which are further categorized by herbicide chemistry group.
Strictly soil applied herbicides are divided into mode-of-action and
then into herbicide chemistry groups.

Plants are complex organisms with well-defined structures in which
multitudes of vital (living) processes take place in well ordered and
integrated sequences. Plants are made up of organs (root, stem, leaf,
and flower); organs consist of tissues (meristems, conducting,
photosynthetic, structural); and tissues are made up of cells. Plant
cells contain subunits including walls, membrane systems (golgi, plasma
membrane, nuclear membrane, endoplasmic reticulum) and organelles
(mitochondria, nucleus, chloroplasts), and undifferentiated cytoplasm.

Some vital metabolic plant processes include photosynthesis (capture of
light energy and carbohydrate synthesis), amino acid and protein
synthesis, fat (lipid) synthesis, pigment synthesis, nucleic acid
synthesis (RNA - DNA essential to information storage and transfer),
respiration (oxidation of carbohydrate to provide CO2 and usable
energy), energy transfer (nucleic acids) and maintenance of membrane
integrity. Other vital processes include growth and differentiation,
mitosis (cell division) in plant meristems, meiosis (division resulting
in gamete and seed formation), uptake of ions and molecules,
translocation of ions and molecules, and transpiration. One or more of
the vital processes must be disrupted in order for a herbicide to kill a
weed.

I. Foliar Applied Herbicides

These herbicides are capable of moving from leaves (sources of sugar
production) with sugars to sites of metabolic activity (sinks of sugar
utilization) such as underground meristems (root tips), shoot meristems
(shoot tips), storage organs and other live tissues. Since movement to
sites is essential for continued plant growth, these herbicides have the
potential to kill simple perennial and creeping perennial weeds with
only one or two foliar applications.

Symptoms are evident on new growth first. Pigment loss (yellow or
white), stoppage of growth, and distorted (malformed) new growth are
typical symptoms. Most injury appears only after several days or weeks.
Plants die slowly. Herbicides in this group are usually molecular (non-
charged) at low pHs found in the cell walls and negatively charged at
higher pHs encountered in the cytoplasm of leaf sieve cells of the
phloem (the ionization inside the cytoplasm of the phloem accounts for
trapping and movement of these herbicides).

1. Auxin Growth Regulators

The effects associated with auxins help set them apart from other
downwardly mobile herbicides. Bending and twisting of leaves and stems
is evident almost immediately after application. Delayed symptom
development includes root formation on dicot stems; misshapened leaves,
stems, and flowers; and abnormal roots.

Soil activity varies from almost none to long residual depending on
herbicide and dose.

Auxin growth regulator herbicides are used for control of annual, simple
perennial, and creeping perennial broadleaves in grass crops (corn,
small grains, sorghum, turf, pastures, sodded roadsides and rangeland)
and in non-crop situations. All are organic acids which take on a
negative charge after ionization of acids and salts. Esters are
hydrolyzed to acids or salts in both plants and soils. Injury to off-target
vegetation is a major problem associated with these herbicides.

Common Trade
Name Name

Phenoxyaliphatic Acid Herbicides

2,4-D
2,4-DB
MCPP (mecoprop)
MCPA
2,4-DP (dichlorprop)

Benzoic Acids

dicamba BANVEL/
CLARITY/
VANQUISH/
VETERAN

Picolinic Acids (Pyridines) and Relatives

2. Amino Acid Inhibitors (Aromatic)

Glyphosate and sulfosate are the compounds with this
mode of action.
Uses are limited to foliar applications only, since these chemicals are
rapidly inactivated in the soil. Symptoms include yellowing of new
growth and death of treated plants in days to weeks. These relatively
nonselective compounds control annual grasses, annual broadleaves,
johnsongrass, quackgrass, yellow nutsedge, cool season pasture and turf
grasses, cattail, Canada thistle, hemp dogbane, Jerusalem artichoke,
poison ivy, and multiflora rose. Glyphosate tolerant cultivars of
soybeans (Roundup Ready), corn, and other crops are currently being
marketed. Corn and other glyphosate tolerant crops are being tested for future release.

Common Trade
Name Name
glyphosate ROUNDUP ULTRA/
RODEO/ACCORD
sulfosate TOUCHDOWN

3. Amino Acid Inhibitors [Branched-chain (AHAS/ALS)]

Several groups of different chemistry have
this same mode of action.
Shoot meristems cease growth; yellow, pink and purple symptoms appear;
roots tend to develop poorly; and the secondary roots are shortened and
all nearly the same length producing a "bottlebrush" appearance.
Complete symptom development is very slow and requires two to three
weeks or more. Late postemergence applications of some of these
herbicides used on corn may result in malformed (bottle shaped) ears.

Imidazolinones

Weed control in soybeans, alfalfa, wheat, barley, and non-crop
situations is the major use of these compounds. Compounds are residual
(weeks) to long-residual (several months) depending on herbicide dose.
Dry weather and cool temperatures in particular and possibly low pH and
high organic matter contribute to persistence in the soil. Imidazolinone
tolerant corn cultivars are being marketed for use with imazethapyr.

Sulfonylureas

Sulfonylurea herbicides are applied preplant incorporated,
preemergence, and postemergence at doses of 0.5 to 6 ounces active
ingredient per acre. This herbicide group provides selective control of
wild garlic and Canada thistle in small grains; broadleaf weeds in
soybeans; johnsongrass, shattercane, quackgrass and wirestem muhly in
corn; and weeds in conifers, hardwoods and pastures. Several compounds
are used for general vegetation control on non-crop sites. High soil pH
greatly increases persistence since only biodegradation takes place at
higher soil pHs. At soil pHs below 6.8, chemical degradation occurs in
addition to biodegradation and speeds inactivation. Sulfonylurea
tolerant soybeans are available to farmers.

Sulfonanilides

Selective soil or foliar applied for control of annual broadleaf weeds
in corn or soil applied treatments in soybeans.

flumetsulam BROADSTRIKE

4. Chlorophyll/Carotenoid Pigment Inhibitors

Vivid white new growth, sometimes tinged with pink or purple,
characterize the symptoms associated with the pigment inhibitors. New
growth initially appears normal except for the conspicuous lack of green
and yellow pigments. Uses include, selective weed control in soybeans
and cotton, poison ivy control, general vegetation control and aquatic
weed control.

Amitrole is the only compound of this group
which moves well in the
symplast, however other compounds in the group show initial movement
into shoot tips causing new growth to be devoid of green and yellow
pigments.

5. Grass Meristem Destroyers (Lipid Biosynthesis Inhibitors)

All provide the same symptoms on grass species;
namely discoloration
and disintegration of meristematic tissue at and above the nodes,
including nodes of rhizomes. Leaves yellow, redden and sometimes wilt.
Seedling grasses tend to lodge by breaking over at the soil. These
herbicides have the potential to be used for selective removal of most
grass species from any non-grass crop. There is also some selectivity
among grass species (particularly with the aryloxyphenoxypropionates in
cool season grasses). The grass meristem destroyers should be used early
postemergence on seedling grasses, and postemergence but before the boot
stage (the seedhead detectable in the top leaf sheath) on established
perennial grasses. Mixing with postemergence broadleaf herbicides
frequently results in reduced grass control. When used under less than
ideal conditions (no-till, open crop canopies and drought) two
applications per season are frequently required.

These compounds are more active postemergence (foliar) than soil
applied. At normally used postemergence doses, soil activity is marginal
or lacking.

Aryloxyphenoxypropionates

Cyclohexanediones

clethodim SELECT
sethoxydim POAST/
POAST PLUS

B. Non Translocated (Contact Herbicides)

Cell Membrane Destroyers

Compounds in this group result in rapid disruption of cell membranes and
very rapid kill. The bipyridyliums and the diphenyl ethers penetrate
into the cytoplasm, cause the formation of peroxides and free electrons
(light is required) which destroy the cell membranes almost immediately.
Herbicidal oils dissolve membranes directly. Rapid destruction of cell
membranes prevents translocation to other regions of the plant. Severe
injury is evident hours after application, first as water-soaked areas
which later turn yellow or brown. Maximum kill is attained in a week or
less. Partial coverage of a plant with spray results in spotting and/or
partial shoot kill. New growth on surviving plants will be normal in
appearance. Foliar activity alone can provide only shoot kill.

Bipyridyliums

These foliar applied, strongly cationic, relatively toxic herbicides are
used postemergence only. Extremely strong binding to clay prevents
activity for weed control or leaching in the soil. Only shoot kill can
be expected. Liquids with suspended colloids (muddy water, slurry
fertilizers) cause inactivation. These herbicides are used for general
shoot kill in numerous situations including burn down in conservation
tillage systems and preharvest desiccation. Diquat is used for control
of aquatic weeds.

Common Trade
Name Name
paraquat GRAMOXONE
diquat DIQUAT/REWARD

Diphenyl ethers (nitrophenyl ethers)

These herbicides have both foliar and soil activity. They mostly control
broadleaves. Acifluorfen is labeled for postemergence applications to
soybeans, peanuts, and rice. Fomesafen and lactofen are similar to
acifluorfen. Although bronzing or burning of soybean leaf tissue is
evident after application, yield is rarely affected. Oxyfluorfen is used
preemergence for cole crops and postemergence for mint, onions and
conifer nurseries. This herbicide group is relatively unaffected by soil
texture and organic matter.

Other postemergence herbicides

Bentazon is used only postemergence in large seeded legumes and some
grass crops for control of annual broadleaf weeds and yellow nutsedge
and shoot removal of perennial broadleaf weeds. This compound inhibits
photosynthesis in the target plant.

Glufosinate is applied postemergence for control of annuals prior to
crop establishment, for noncrop areas and for selective directed
placement in specialty crops (apples, grapes, tree nuts). There is no
soil activity. The inhibition of the glutamine synthetase enzyme in the
effected plant results in the decrease of several amino acids which
eventually leads to cell membrane disruption and death of the cell.
Symptoms of the plant include chlorosis (yellowing) followed by necrosis
(dead tissue) 3 to 5 days after herbicide application. Glufosinate
tolerant cultivars of rice, soybeans, and corn are being tested.

Common Trade
Name Name
bentazon BASAGRAN
glufosinate IGNITE/RELY/
FINALE/LIBERTY

C. Upwardly Mobile Only Herbicides (Apoplastically Translocated)

Photosynthetic Inhibitors

These herbicides translocate only apoplastically. Movement is upward
with the transpiration stream (water moving through the plant from the
soil and evaporating into the atmosphere at the leaf surfaces).

Symptoms develop from bottom to top on plant shoots (older leaves show
most injury; newer leaves least injury). Chlorosis first appears between
leaf veins and along the margins which is later followed by necrosis of
the tissue. Any potential control of established perennials must come
from continued soil uptake and not movement downward through the plant
from the shoots. Foliar activity alone can provide only shoot kill.

Herbicides in these chemical groups have excellent soil activity. Most
have foliar activity as well. These herbicides are used preplant
incorporated, preemergence, and to a limited extent early postemergence,
for selective control of weeds in annual and established perennial
crops. Crops include corn, soybeans, potatoes, celery, parsnips,
carrots, cotton, alfalfa, asparagus, mint, and woody species. They are
also used for brush in pastures, rangeland, and non-cropland and for
general vegetation control. Soil persistence varies from weeks to months
depending on compound and dose and soil pH. Soil mobility varies from
low to high depending on the compound and soil characteristics.

Triazines

Major herbicides for weeds in corn, they are also used in sorghum, numerous
woody species, and for total vegetation control. Use for aquatics has been
discontinued. Detection in and public concern regarding surface and ground water may result in severe restrictions on use of the triazine herbicides.

Uracils

Phenylureas

linuron LOROX/LINEX
diuron KARMEX
tebuthiuron SPIKE

Others (not typical)

bentazon BASAGRAN
bromoxynil BUCTRIL
pyridate TOUGH/LENTAGRAN

II. Soil Applied Herbicides

Cell Division Inhibitors

Root Inhibitors

These herbicide groups have little or no foliar activity and are applied
mostly preplant incorporated and preemergence for control of seedling
grasses and some annual broadleaves in soybeans, peanuts, dry beans,
cole crops, cotton, alfalfa, clovers, lettuce, tobacco, herbaceous
ornamentals, established turf, and in woody species (nurseries,
orchards, grapes, Christmas trees, etc.).

Dinitroanilines (Dinitrobenzenamines)

These herbicides inhibit the steps in plant cell division responsible
for chromosome separation and cell wall formation. Roots are relatively
few in number and club shaped. Except for oryzalin, these compounds have
water solubility less than one part per million. They bind to soil
colloids and are unlikely to leach. Losses occur through volatilization
and photodegradation on soil surfaces. Incorporation into the soil by
mechanical mixing or by overhead irrigation soon after application is
routinely suggested. These root inhibitors do not translocate.

Miscellaneous Herbicides

DCPA is labeled soil applied for seedling grass control in large seeded
legumes, cotton, cole crops, onions, garlics, potatoes, other vegetables,
established turf, herbaceous ornamentals, and small fruits.

Siduron is labeled soil applied for seedling grass control in newly seeded
or newly sprigged turf and established turf. It removes annual grass competition from spring established turf.

Common Trade
Name Name
DCPA DACTHAL
siduron TUPERSAN

2. Shoot Inhibitors

The shoot inhibitors are soil applied for control of seedling grasses,
some broadleaves and suppression of some perennials from tubers and
rhizomes. Injury appears as malformed (twisted), dark green shoots and
leaves on injured young plants. Grass crops with some tolerance to these
compounds can be protected from injury with other chemicals [safeners
(protectants)]. Crops include corn, large seeded legumes, small seeded
legumes, beets, spinach, tomatoes, potatoes, and ornamentals.

Thiocarbamates (Carbamothioates)

This group of very volatile herbicides is used preplant incorporated.
They persist in the soil for two to six weeks and are particularly
effective for control of seedling grasses including johnsongrass and
shattercane.

Substituted Amides (Chloroacetamides)

These are the major preemergence herbicides for seedling grass control
in corn and soybeans in the Eastern Corn-belt. Several provide decent
control of seedling grasses in higher organic matter soils. Most are
labeled for preplant incorporated application. Most of these herbicides
control yellow nutsedge and black nightshade. Typical persistence in the
soil is 10 to 15 weeks.

3. Shoot and Root Inhibitors

Preplant incorporated, preemergence and sometimes early postemergence
for control of annual grasses, and some annual broadleaves in small
seeded legumes, lettuce, established woody species, established turf,
strawberries, established herbaceous perennials, tomatoes, cole crops,
cotton, cucurbits, peppers, and tobacco.

Rev 4/96Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University
and U.S. Department of Agriculture Cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued
in furtherance of the Acts of May 8 and June 30, 1914. Purdue University Cooperative Extension
Service is an equal opportunity and access institution.